I appreciate the response! My understanding is that the defense calculation will target whatever your highest defense type is. In the case of an attack that does 80% cold and 20% smashing, your smashing defense will be used to calculate whether the attack hits if that's your higher stat. If this isn't the case, please let me know!
That said, it's not about defense being the only thing attributing to the diversity. It's about the potential solutions to adversity. Currently, your most viable option to the game is to cap out your Smashing/Lethal/Energy defenses and you're set to handle a majority of the content. Why would you try to put defenses anywhere else when the other option is statistically superior? With this change in place, capping out those defenses isn't a one-size-fits-all scenario, so I feel more free to focus in other areas of my character. Will I be as powerful as I would be without this change? Definitely not, but I have other solutions whether it be attempting to max out other defenses, teaming up, or changing my general tactics depending on the enemy group. These other options are what I meant when I said it contributes to diversity.
In terms of freedom for the developers, this system seems like it'll give developers more dials and nobs to tweak when making new content. Creating an attack for an enemy has implications for resistance based characters just as much as it does for defense based ones. Designing an enemy that does Cold damage would be great for Ice Armor, but could potentially more powerful against other sets such as Fiery Aura. A way to try to balance it is to add one of those would be to add one of those common damage types, such as Smashing. But even if the Smashing component was only 10% in the current system, a typed defense based character would get the maximum benefit from their Smashing defense alone. It doesn't give the designer the luxury of designing this enemy without other implications, they can tweak the percentage anywhere between 1% and 99% Smashing to deal with resistance based characters, but not the typed defense one. With the new system, the designer can tweak it up to 49% before changes start taking effect. Not only do I think this makes sense mechanically, this makes sense to me narratively as well. Why would a character know how to perfectly defend against an attack even if it does only 1% of the damage your character specializes in defending against?
To me, this is really the same kind of change as the Enhancements Diversification. I understand that people were saddened by the change in power to their characters, but it gave them the ability to shine in other areas and be just as viable. In the content they couldn't solo anymore with their old builds, there were still alternatives. I think the number of alternatives, how they interact, and what we see in team compositions is what I'm excited for by this change.